Process and apparatus for separating fluid mixtures



Nov. 6, 1956 R. l.. IRVINE 2,769,309

PROCESS AND APPARATUS FOR SEPARATING FLUID MIXTURES Filed Aug. 7, 1952PROCESS AND APPARATUS non SEPARATING FLUID MlXrUnES lThis inventionrelates to fractional distillation and more particularly to a method andapparatus for the fractional distillation of mixtures of narrow boilingrange. p

`vMixtures of narrow boiling range present a difcult separation problem.A conventional method of frac,- tionating such mixtures involves the useof high temperatures and pressures in the fractionating Acolumn and thecooling of the overhead product by available cooling water. Because ofthe relatively high temperature of normal cooling water, e. g. 85 F., itis necessary for the fractionating column pressure to be relatively highinorder to condense the overhead product. Al-so, a khigh reux ratio isrequired in separating a narrow `boiling point mixture at hightemperature and pressure, and this contributes to the excessive cost ofsuch operation. It has been recognized thatkthe refrigeration of theoverhead condenser would be desirablepfor separating narrow boilingrange mixtures so that the column could lbe maintained at lowtemperatures and pressures, but the `use of conventional compressionrefrigeration equipment to obtain a lower condensing temperature thanordinary cooling water provides is not usually economically practicable.v

The present invention makes it possible to refrigerate the overheadcondenser of Ka fractionating column at operating costs not exceedingthose of columns using cheap cooling water. As a consequence, thefractionationV of narrow boiling range mixtures such aspropylene-propane or isobutane-n-butane is greatly improved becauseofthe lower permissible temperatures and pressures in the fractionatingcolumn which increase the differences inl relative volatilities of thelighter to heavier components and thus decrease the required work ofseparation.

A detailed description of the invention can be given by reference to theaccompanying drawing which is a diagrammatic representation of apreferred embodiment of the apparatus of the invention in which theprocess of the invention can be carried out. v

In the apparatus of the drawing, material to be fractionated iscontinuously introduced through line 1 into fractionating column 2provided with suitable liquidvapor contacting means, e. g. bubble captrays or perforated trays, and is subjected to conventionalfractionation therein. The more volatile fraction of the charge mixtureis continuously withdrawn as a vapor from the top of the column throughline 3. The overhead product vapors are continuously cooled and at leastpartially condensed in a refrigerated overhead product condenser byindirect heat exchange with an evaporating refrigerant. The drawingshows the preferred embodiment of my apparatus in which I employ aconventional overheadcondenser 4 and refrigerate this condenser bypassing through its tubes a stream of chilled liquid delivered from arefrigerant evaporating zone.

The condensate from condenser 4 and any remaining uncondensed materialpass through line 5 into retlux drum 6 in which uncondensed material isdisengaged United States Patent ice liatented Nov. 6,1956

from the condensate. The non-condensable vapors, if any, :are ventedfrom reilux ydrum 6 through line 7 and disposed of as desired.Condensate is withdrawn through line 8 and pump 9. A portion of thecondensate, sufcient for reuxing, is returned via line 10 to the top ofthe fractionating column 2 while the remainder is withdrawn via line 11as overhead product.

ln accordance with my preferred procedure, lthe same liquid is usedboth` as the evaporating refrigerantuand as the chilled liquid forpassage through condenser 4. It is also preferred that this liquid bewater. The advantages of using water for both of these purposes will beapparent from the further description of the processand apparatus of thedrawing in which water is used for both purposes. H

The chilled water introduced by line 12 for refrigerating condenser 4 isreturned by line 13 for rechilling and is sprayed through header 14 intothe evaporation zone in the upper portion of shell 15. At the start ofthe process a partial vacuum is imposed on the shell 15k by any suitablevacuum producing means such as a steam jet, not shown in the drawing, soas to promote the vaporization of the water introduced into the`evaporation zone of shell 15. A portion of the water sprayed into theupper portion of shell 15 flashes into vapor and thus chills theremaining liquid down to a temperature nearly corresponding to thetemperature of water with vapor pressure equal to the pressure in theshell. The Water chilled in this manner collects in evaporator tank lofrom which it drains by line` 17 and is returned by chilled water pump18 to theoverhead condenser 4.

A concentrated salt solution, preferably an aqueous solution of lithiumbromide, is introduced through sprayer 19 into the lower portion of thelow-pressure shell 15. The water vapor dashed in the evaporation zone ofshell 15 moves down and is absorbed by the concentrated salt solution sothat the shell pressure remains below the vapor pressure of the waterintroduced through header 14 and continuous evaporation of the wateroccurs. To lassist in Vthe maintenance of the pro'per low pressure inshell 15, the steam jet used for producing the original vacuum in t-heVshell can function to purge the shell of any non-condensables whichmight enter the shell by leakage. A

Since the heats of condensation and dilution .resulting from theabsorption of water Vapor in the lithium bromide solution tend to raisethe temperature of the salt solution and reduce its labsorptive power,it is preferred to provide cooling means in the shell 15 such as thecoil 20, over which the salt solution is sprayed. Through coil 20 ispassed a coolant such as cooling water to absorb the heat released inthe vapor absorption.

The absorption of water vapor in the salt solution lowers its saltconcentration and thus reduces its absorbing power. Diluted saltsolution is continuously drained from shell 15 and is deliveredby,dilute solution pump 21 to generator 22. In generator 22, a heatingmedium such as exhaust steam passing through coil 23 heats the solutionand boils olf the water vapor previously condensed in the adsorptionzone of shell 15, thus restoring the solution to its originalconcentration. lt is preferred to impose a partial vacuum on thegenerator 22 so that generation of vapors from the diluted absorbentsolution will not require an excessively high temperature. The partialvacuum can be imposed in any suitable manner, for example as in shell15, by means of a conventional steam jet.

Water vapor boiled out of the solution in generator 22 passes via lines24 and 25 into the tubes of reboiler 26 and liquid from the bottom ofcolumn 2 is passed through the reboiler shell. In this manner, by heateX- change between the generated water vapor and the column bottomliquid the water vapor from generator 22 is condensed and the reboilingof column 2 is effected. Condensed water vapor is returned to theevaporator tank 16 via line 27. Preferably, as shown in the drawing, aheat exchanger 28 is placed between the pump 21 and the generator 22 tocool the hot concentrated solution withdrawn from the bottom ofgenerator 22 and to heat the cooler diluted solution entering generator22.

In the preferred embodiment of my invention the heat balance in thecolumn 2 is maintained through the use of the balancing condenser 30.The coolant line passing through condenser 30 is provided with a owcontrol valve 31. The valve 31 is operated by a pressure controller atthe lower end of column 2. When the pressure in column 2 tends to riseabove a predetermined pressure level, the pressure controller operatesvalve 31 to increase the rate of flow of coolant through the tubes ofcondenser 30. As a result the rate of flow of Vapor from line 24 throughline 29 into the condenser 30 increases so that the flow of vaporthrough the reboiler decreases and the temperature of column 2 drops.Likewise, when pressure tends to drop in column 2 the temperature of thecondenser 30 is increased by a decrease in the coolant rate, and thevapor flow increases through line 25 to the reboiler and therefore thetemperature in the bottom of column 2 rises.

From the foregoing description it can be seen that my invention makes itpossible to refrigerate the overhead condenser of a fractionating columnand obtain the im portant advantages resulting therefrom at operatingcosts not exceeding the operating costs for a fractionating column whichuses normal temperature cooling water in its overhead condenser. Theapparatus of my invention is operated by the same heat, e. g. exhauststeam, as would be supplied to a conventional fractionating column, butthe heat is supplied to the generator 22 of the refrigeration equipmentrather than directly to the fractionating column reboiler as inconventional columns. Then, since the column temperature can be low as aresult of refrigeration of the overhead condenser, the reboilertemperature can be lower so that for this purpose the heat supplied bycondensing the vapor from generator 22 is sufficient. Thus, forsubstantially the same operating cost as conventional fractionation, itis possible to operate a fractionating column in accordance with myinvention at a considerably lower temperature and pressure thanconventional columns. The resulting advantages are numerous.

One advantage is that my invention makes it possible to use existingfractionating columns for fractionations which by conventional practicewould take place at pressures higher than the existing equipment couldwithstand. For example, a conventional depentanizer which uses coolingwater in its overhead condenser operates at about 25 pounds per squareinch gauge. A conventional debutanizer column also using cooling Waterin its overhead condenser operates at about 100 pounds per square inchgauge and must, of course, be of considerably heavier construction thanthe depentanizer column. My invention makes it possible to use anexisting depentanizer column as a debutanizer since the debutanizer canoperate at pressures as low as about 25 pounds per square inch gaugewhen the overhead condenser is refrigerated with chilled water inaccordance with my invention. Another advantage is that with lowerpressure and temperature the same degree of separation of a narrowboiling range mixture can be obtained with a much lower refiux ratiothan is required in a column of higher temperature and pressure. Becauseof the lower reflux ratio, a smaller column can handle the same amountof charge.

My invention affords still other advantages in certain separations. Forexample, in separating a mixture of isobutane and n-butane in accordancewith my invention, the bottoms from the fractionating column is obtainedat a considerably lower temperature than in conventional towers, namelyat about F., so that it is unnecessary to cool the bottoms productbefore using or storing it. This makes it possible to eliminate one ofthe heat exchanges of conventional towers. Also, in the separation ofthis mixture to obtain isobutane for alkylation, the isobutane overheadproduct is chilled by the overhead condenser to a rather lowtemperature, about 45 F., so that further chilling of the isobutane asrequired in columns in which the overhead condenser employs ordinarycooling water, is unnecessary before charging the isobutane to thealkylation unit.

As an illustration of the type of results obtainable with the processand apparatus of my invention the following example can be given.

Example To a fractionating column of the type shown in the drawing ischarged a mixture comprising npropane, 1 percent by Volume; isobutane 22percent by volume; n-butane 76 percent by volume and isopentane lpercent by volume. Exhaust steam at 230 F. is delivered to the generator22 of the refrigerating equipment. An aqueous solution of lithiumbromide is employed as the refrigerating system absorbent. Theconcentration of the lithium bromide solution varies from about 60 toabout 65 percent during the operation. The temperature of overheadcondenser 4 is maintained at about 50 F. by circulating through itstubes chilled water delivered from the refrigeration equipment at about41 F. The condenser temperature of 50 F. permits temperatures in thefractionating column ranging from 60 F.. at the top of the column to 85F. in the reboiling section at the bottom of the column. The pressure ismaintained at about 40 pounds per square inch absolute at the bottom ofthe column. With a 9 to 1 reflux ratio the overhead product of thecolumn has the composition: n-propane, 3.8 volume percent; isobutane91.3 volume percent; and n-butane 4.9 volume percent. The bottomsproduct has the composition: isobutane 2.3 volume percent; n-butane 96.7volume percent; and isopentane 1.0 volume percent.

In the conventional fractionation of the same C4 mixture, the overheadcondenser would be cooled by available cooling water at about 85 F. andthe overhead condenser temperature would be about F. The conditions ofoperation of the column in order to obtain a degree of separationcomparable to the separation in my process would include the following:temperature at the top of the tower, about 135 F. and at the bottom ofthe tower in the reboiling section, about 230 F.; pressure, about poundsper square inch absolute; reflux ratio, about l2 to 1. From these guresit can clearly be seen that the conventional process would require amuch more expensive fractionating column to withstand the considerablyhigher pressure and to afford the capacity for the considerably higherreflux ratio.

In the detailed description of the process and apparatus of myinvention, it has been stated that the process of the invention employswater as the refrigerant and as the liquid medium for heat exchangebetween the evaporation zone and the overhead condenser. It should beunderstood, however, that while this is the preferred method ofoperation, other procedures can be used. Thus, for example, it ispossible to condense the overhead product in tubes passing through theevaporation zone, in effect making the evaporation zone serve directlyas the overhead condenser and eliminating the need for delivering achilled liquid from the evaporation zone to a separate overheadcondenser.

Another possible modification of the process is the employment of a heatexchange medium other than water for refrigerating the separate overheadcondenser. Thus it is possible to provide coils in the evaporation zonethrough which brine or other heat transfer medium is flowed and, afterchilling, delivered to the overhead condenser. In such case the brinestream would be a separate closed system and the cooling effect would beobtained by evaporation of condensate returned to the evaporation zoneby line 27.

It is also possible in my process to employ substances other than thepreferred lithium bromide as the absorbent. lt is, of course, importantthat the absorbent be a solid which has little or no vapor pressure ofits own and most of the suitable solids are salts. The absorbent shouldform a low viscosity solution so as to minimize the Work of pumping andthe solution should, of course, be of low corrosivity. The absorbentshould have a low heat of dilution and concentration and should havegood heat transfer characteristics such as high thermal conductivity andhigh specific heat. A primary requirement of the absorbent is a strongabsorbing affinity for the refrigerant liquid used in the process.Compounds having the indicated desired properties includes the varioushalides of alkali metals and alkaline earth metals.

While water is greatly preferred as the liquid refrigerant because ofits cheapness and its many desirable properties, the invention alsoincludes the use of other liquids. Ethylene glycol is an example of aliquid which is suitable with certain absorbent salts.

Obviously many modifications and variations of the invention ashereinbefore set forth may be made without departing from the spirit andscope thereof; therefore only such limitations should be imposed as areindicated in the appended claims.

I claim:

l. The process which comprises subjecting a narrow boiling range uidmixture to fractionation in a fractionating column, continuouslyremoving overhead the more volatile fraction of the mixture as a vapor,cooling and at least partially condensing such vapors by heat exchangewith a stream of chilled liquid refrigerant delivered from anevaporation zone, imposing a pressure in said evaporation Zone below thevapor pressure of said liquid refrigerant, chilling said liquidrefrigerant stream by partial evaporation of the liquid in saidevaporation zone, maintaining the pressure in said evaporation zonebelow the vapor pressure of the liquid refrigerant by continuouslyabsorbing the vapors of said refrigerant in a stream of absorbent saltsolution, thus diluting said solution, passing said stream of dilutedabsorbent salt solution from the zone of absorption to a generatingzone, heating the diluted absorbent salt solution in said generatingzone to generate refrigerant vapor and reconcentrate said solution,returning the reconcentrated solution to the zone of absorption,exchanging heat between at least a portion of said refrigerant vapor andthe bottom liquid of said fractionating column whereby to condense saidrefrigerant and reboil said column, passing another portion of saidrefrigerant vapor-into heat exchange with a coolant in a separate heatexchanging zone, controlling the rate of flow of coolant to saidseparate heat exchanging zone in accordance with pressure variationswithin said fractionating column whereby to increase the rate of ow ofcoolant to said separate heat exchanging zone when the pressure in saidfractionating column tends to rise above a predetermined pressure level,and returning the condensed refrigerant to the zone of evaporation.

2. In a fractional distillation apparatus, the combination of afractionating column and a heat operated absorption refrigerationapparatus, means for condensing the overhead vapors from saidfractionating column by indirect heat exchange with chilled refrigerantfrom said refrigeration apparatus, means for delivering chilled liquidfrom a refrigerant evaporator to said condensing means, means forreturning liquid refrigerant from said condensing means to saidevaporator for partial vaporization of said refrigerant, means forintroducing an absorbent solution in the region of said evaporator toabsorb the vaporized refrigerant thus diluting the absorbent solution,means for delivering diluted absorbent solution to a refrigerantgenerator, means for heating said generator to form refrigerant vaporand reconcentrate said solution, means for delivering refrigerant vaporfrom said generator to the reboiler of said fractionating column forindirect heat exchange with the bottoms liquid of said column, means fordelivering a portion of the vapors from said generator to a condensingmeans, means for delivering a coolant to said condensing means andhaving a ow control valve, and pressure responsive means in the lowerend of said fractionating column for controlling said valve to increasethe rate of coolant passing through said condensing means when thepressure in said column tends to rise above a predetermined maximum.

3. The process which comprises subjecting a liuid mixture tofractionation in a fractionating column, continuously removing overheadthe more volatile fraction of the mixture as a vapor, cooling and atleast partially condensing such vapor by heat exchange with anevaporating refrigerant, absorbing the vapors of said refrigerant in anabsorbent solution, thus diluting the solution, heating the dilutedsolution to generate refrigerant vapor and reconcentrate said solution,exchanging heat between at least a portion of said refrigerant vapor andthe bottoms liquid of said fractionating column whereby to condense saidrefrigerant and reboil said column, passing another portion of saidrefrigerant vapor into heat exchange with a coolant in a separate heatexchanging zone, and controlling the rate of ow of coolant to saidseparate heat exchanging Zone in accordance with pressure variationswithin said fractionating column whereby to increase the rate of flow ofcoolant to said separate heat exchanging zone when the pressure in saidfractionating column tends to rise above a predetermined pressure level.

4. ln 4a fractional distillation apparatus, the combination of afractionating column and a heat operated absorption refrigerationapparatus, means for condensing the overhead vapors from saidfractionating column by indirect heat exchange with chilled refrigerantfrom said refrigeration apparatus, means for reboiling said column byindirect heat exchange between the bottoms liquid of said column andregenerated refrigerant vapors from said refrigeration apparatus, aseparate condensing means, means for delivering a portion of thegenerated refrigerant vapors from said refrigeration apparatus to saidseparate condensing means7 means for delivering a coolant to saidseparate condensing means and having a flow control valve, and pressureresponsive means in the lower end of said fractionating column forcontrolling said valve to increase the rate of coolant passing throughsaid condensing means when the pressure in said column tends to riseabove a predetermined maximum.

References Cited in the file of this patent UNITED STATES PATENTS2,127,004 Nelson Aug. 16, 1938 2,182,098 Sellew Dec. 5, 1939 2,284,662Kahle June 2, 1942 2,336,097 Hutchison Dec. 7, 1943 2,461,513 BerestneffFeb. l5, 1949 2,534,274 Kneil Dec. 19, 1950

